Electro-hydraulically controlled variable assist power steering systems are known. For example, U.S. Pat. No. 4,760,892 describes such a system in which a variable degree of power assistance is provided to supplement the manual steering effort by controlling the magnitude of the steering pressure made available to a pressure operated motor by the vehicle's power steering pump. As described, the magnitude of the pressure is controlled by a central processing unit that responds to vehicle steering torque and vehicle speed to produce an appropriate steering pressure and power assist for all driving conditions.
However, in prior systems, a mechanical steering valve has been relied upon as the direction control valve for determining the degree of communication between the steering pressure and the hydraulic motor. This type of system has a number of limitations. For one thing, the mechanical steering valve has inherent in its operation a certain amount of backlash or a dead band in which, although the vehicle steering wheel may be turned, no power assist is provided. In addition, with mechanical valves, rapid turning of the steering wheel results in hydraulic shock loading of the mechanical and hydraulic components of the steering system. In addition, since the steering valves are mechanical rotary hydraulic control valves, their operational characteristics are not easily changeable so that it is difficult to tailor their characteristics to a particular vehicle, and the same valve may not be easily adaptable from one vehicle to a different type of vehicle.